GenericFunctions {methods} | R Documentation

## Tools for Managing Generic Functions

### Description

The functions documented here manage collections of methods associated with a generic function, as well as providing information about the generic functions themselves.

### Usage

    isGeneric(f, where, fdef, getName = FALSE)

    isGroup(f, where, fdef)

    removeGeneric(f, where)

    standardGeneric(f)

    dumpMethod(f, signature, file, where, def)

    findFunction(f, generic=TRUE)

    dumpMethods(f, file, signature, methods, where)

    signature(...)

    removeMethods(f, where)

    setReplaceMethod(f, ...)

    getGenerics(where, searchForm = FALSE)

    allGenerics(where, searchForm = FALSE)

    callGeneric(...)

### Arguments

`f` | The character string naming the function.

`where`

The environment, namespace, or search-list position from which to search for objects. By default, start at the top-level environment of the calling function, typically the global environment (i.e., use the search list), or the namespace of a package from which the call came. It is important to supply this argument when calling any of these functions indirectly. With package namespaces, the default is likely to be wrong in such calls.

`signature` | The class signature of the relevant method. A signature is a named or unnamed vector of character strings. If named, the names must be formal argument names for the generic function. If `signature` is unnamed, the default is to use the first `length(signature)` formal arguments of the function.
`file` | The file on which to dump method definitions.
`def` | The function object defining the method; if omitted, the current method definition corresponding to the signature.
`...` | Named or unnamed arguments to form a signature.
`generic` | In testing or finding functions, should generic functions be included. Supply as `FALSE` to get only non-generic functions.
`fdef` | Optional, the generic function definition. Usually omitted in calls to `isGeneric`
`getName` | If `TRUE`, `isGeneric` returns the name of the generic. By default, it returns `TRUE`.
`methods` | The methods object containing the methods to be dumped. By default, the methods defined for this generic (optionally on the specified `where` location).
`searchForm` | In `getGenerics`, if `TRUE`, the `package` slot of the returned result is in the form used by `search()`, otherwise as the simple package name (e.g, `"package:base"` vs `"base"`).

### Summary of Functions

`isGeneric`:
  Is there a function named `f`, and if so, is it a generic?

  The `getName` argument allows a function to find the name from a function definition. If it is `TRUE` then the name of the generic is returned, or `FALSE` if this is not a generic function definition.

  The behavior of `isGeneric` and `getGeneric` for primitive functions is slightly different. These functions don't exist as formal function objects (for efficiency and historical reasons), regardless of whether methods have been defined for them. A call to `isGeneric` tells you whether methods have been defined for this primitive function, anywhere in the current search list, or in the specified position `where`. In contrast, a call to `getGeneric` will return what the generic for that function would be, even if no methods have been currently defined for it.

`removeGeneric`, `removeMethods`:
  Remove the all the methods for the generic function of this name. In addition, `removeGeneric` removes the function itself; `removeMethods` restores the non-generic function which was the default method. If there was no default method, `removeMethods` leaves a generic function with no methods.
`standardGeneric`:
  Dispatches a method from the current function call for the generic function `f`. It is an error to call `standardGeneric` anywhere except in the body of the corresponding generic function.
`getMethods`:
  The list of methods for the specified generic.
`dumpMethod`:
  Dump the method for this generic function and signature.
`findFunction`:
  return a list of either the positions on the search list, or the current top-level environment, on which a function object for `name` exists. The returned value is always a list, use the first element to access the first visible version of the function. See the example.

  NOTE: Use this rather than `find` with `mode="function"`, which is not as meaningful, and has a few subtle bugs from its use of regular expressions. Also, `findFunction` works correctly in the code for a package when attaching the package via a call to `library`.

`selectMethod`:
  Returns the method (a function) that R would use to evaluate a call to this generic, with arguments corresponding to the specified signature.

  `f` = the name of the generic function, `signature` is the signature of classes to match to the arguments of `f`.

`dumpMethods`:
  Dump all the methods for this generic.
`signature`:
  Returns a named list of classes to be matched to arguments of a generic function.
`getGenerics`:
  Returns the names of the generic functions that have methods defined on `where`; this argument can be an environment or an index into the search list. By default, the whole search list is used.

  The methods definitions are stored with package qualifiers; for example, methods for function `"initialize"` might refer to two different functions of that name, on different packages. The package names corresponding to the method list object are contained in the slot `package` of the returned object. The form of the returned name can be plain (e.g., `"base"`), or in the form used in the search list (`"package:base"`) according to the value of `searchForm`

`callGeneric`:
  In the body of a method, this function will make a call to the current generic function. If no arguments are passed to `callGeneric`, the arguments to the current call are passed down; otherwise, the arguments are interpreted as in a call to the generic function.

### Details

`setGeneric`:
  If there is already a non-generic function of this name, it will be used to define the generic unless `def` is supplied, and the current function will become the default method for the generic.

  If `def` is supplied, this defines the generic function, and no default method will exist (often a good feature, if the function should only be available for a meaningful subset of all objects).

  Arguments `group` and `valueClass` are retained for consistency with S-Plus, but are currently not used.

`isGeneric`:
  If the `fdef` argument is supplied, take this as the definition of the generic, and test whether it is really a generic, with `f` as the name of the generic. (This argument is not available in S-Plus.)
`removeGeneric`:
  If `where` supplied, just remove the version on this element of the search list; otherwise, removes the first version encountered.
`standardGeneric`:
  Generic functions should usually have a call to `standardGeneric` as their entire body. They can, however, do any other computations as well.

  The usual `setGeneric` (directly or through calling `setMethod`) creates a function with a call to `standardGeneric`.

`getMethods`:
  If the function is not a generic function, returns `NULL`. The `f` argument can be either the character string name of the generic or the object itself.

  The `where` argument optionally says where to look for the function, if `f` is given as the name.

`dumpMethod`:
  The resulting source file will recreate the method.
`findFunction`:
  If `generic` is `FALSE`, ignore generic functions.
`selectMethod`:
  The vector of strings for the classes can be named or not. If named, the names must match formal argument names of `f`. If not named, the signature is assumed to apply to the arguments of `f` in order.

  If `mustFind` is `TRUE`, an error results if there is no method (or no unique method) corresponding to this signature. Otherwise may return `NULL` or a MethodsList object.

`dumpMethods`:
  If `signature` is supplied only the methods matching this initial signature are dumped. (This feature is not found in S-Plus: don't use it if you want compatibility.)
`signature`:
  The advantage of using `signature` is to provide a check on which arguments you meant, as well as clearer documentation in your method specification. In addition, `signature` checks that each of the elements is a single character string.
`removeMethods`:
  Returns `TRUE` if `f` was a generic function, `FALSE` (silently) otherwise.

  If there is a default method, the function will be re-assigned as a simple function with this definition. Otherwise, the generic function remains but with no methods (so any call to it will generate an error). In either case, a following call to `setMethod` will consistently re-establish the same generic function as before.

### References

The R package methods implements, with a few exceptions, the programming interface for classes and methods in the book Programming with Data (John M. Chambers, Springer, 1998), in particular sections 1.6, 2.7, 2.8, and chapters 7 and 8.

While the programming interface for the methods package follows the reference, the R software is an original implementation, so details in the reference that reflect the S4 implementation may appear differently in R. Also, there are extensions to the programming interface developed more recently than the reference. For a discussion of details and ongoing development, see the web page http://developer.r-project.org/methodsPackage.html and the pointers from that page.

### See Also

`setGeneric`, `setClass`, `showMethods`

### Examples

    ## Don't run:
      ## get the function "myFun" -- throw an error if 0 or > 1 versions visible
      allF <- findFunction("myFun")
      if(length(allF) == 0)
        stop("No versions of myFun visible")
      else if(length(allF) > 1)
        stop("myFun is ambiguous: ", length(allF), " versions")
      else
        fdef <- get("myFun", allF[[1]])
    ## End Don't run

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